1. Field of the Invention:
The invention relates to bridge amplifiers, and more particularly, to bridge amplifiers having: (1) linear output response to large transducer deviations; and (2) low output sensitivity to input offset voltages and input offset voltage thermal drift of amplifiers contained in the bridge amplifier circuitry.
2. Description of the Prior Art:
Transducer bridges are widely used to sense deviations or variations in resistance in many types of transducers. One of the most common applications for transducer bridges is in the area of sensing transducer deviation of strain gauges. Another typical application is for sensing photoresistive transducers which are responsive to varying light intensities. Numerous transducer bridges and bridge amplifier circuits (also referred to herein as bridge monitor circuits) are well known. Unfortunately, most of the known bridge amplifier circuits have linear output response only for a very small transducer deviation. Consequently, the known bridges and bridge amplifier circuits require expensive, specialized compensation networks in order to provide more linear operation. This results in undue circuit complexity, cost, and loss of reliability.
It is therefore an object of the invention to provide a low cost bridge amplifier circuit having a linear output response to transducer deviation for a substantially wider range of transducer deviation than the bridge amplifiers of the prior art.
One known bridge amplifier produces an output voltage which is directly proportional to the transducer deviation even for large fractional changes in the transducer resistance. This bridge amplifier circuit is disclosed in FIG. 6.10 of "Operational Amplifiers; Design and Applications", authored by the present inventor, G. Tobey, and L. Huelsman, McGraw-Hill, 1971, and incorporated herein by reference to show the state of the art for bridge amplifiers and for operational amplifiers, which are commonly used in bridge amplifiers. Unfortunately the above mentioned linear bridge amplifier circuit has extremely low gain, so that a second stage amplifier is required, and it requires modification of the bridge configuration, so present commercially available bridges cannot be used.
It is therefore another object of the present invention to provide a high gain bridge amplifier having linear output response to large deviations in transducer resistance.
The above mentioned linear wide-deviation bridge amplifier, when utilized with a subsequent amplifying stage to obtain the amount of gain necessary for most applications, has a high sensitivity to the input offset voltages of both of the operational amplifiers required therein, and consequently also has high sensitivity to thermal drift of the input offset voltage of such operational amplifiers. Compensation for the input offset voltages is expensive, and does not effectively avoid the effects of thermal drift of the input offset voltages. Use of chopper-stabilized operational amplifiers are required in the known bridge amplifier circuits in order to obtain low thermal drift response. Suitable chopper-stabilized amplifiers cost approximately $85.00 apiece at the present time. Therefore, bridge amplifiers which are both linear for large transducer deviations and which have low sensitivity to thermal input offset voltage drift are exceedingly, and in many cases, prohibitively expensive. There is clearly an unmet need for a low cost, linear, drift insensitive bridge amplifier.
One prior patent, U.S. Pat. No. 3,651,696, discloses a non-linear bridge circuit designed to produce a linear output response to temperature variations in a platinum resistance thermometer. Platinum resistance thermometers have a highly non-linear temperature-to-transducer deviation relationship. The circuit described in U.S. Pat. No. 3,651,696, provides circuitry to produce a non-linear response of the bridge amplifier output to transducer resistance deviation of the platinum resistor in order to obtain a linear output response with respect to temperature. The structure of the bridge amplifier is similar in certain respects to that of the presently disclosed invention, but the patent teaches away from the present invention by producing a non-linear, rather than a linear output response to deviations in transducer resistance. Further, the structure taught lacks the desired reduced drift sensitivity.
Other patents uncovered by a novelty search directed to the present invention but deemed less relevant to the present include U.S. Pat. Nos.: 3,597,676; 3,754,442; 3,817,104; 3,887,864; 3,986,393; and, 4,060,714.